1. Field of the Invention
The present invention relates to an organic electroluminescent display device, and more particularly, to an organic thin film transistor including a drain electrode used as a pixel electrode and an organic electroluminescent display device using the same.
2. Description of the Related Technology
Organic thin film transistors (OTFTs) have been recognized as next generation display devices and a great deal of research concerning OTFTs has been conducted. An OTFT includes a semiconductor layer formed of an organic compound instead of silicon. The organic compound can be a low molecular weight organic material or a polymer organic material. The low molecular weight organic material may include oligothiophene, pentacene, or the like. The polymer organic material may include polythiophene or the like.
FIG. 1 is a plan view of a pixel of a flexible organic electroluminescent display device 10 including a conventional OTFT.
Referring to FIG. 1, in the conventional flexible organic electroluminescent display device 10, a pixel is disposed in a pixel region 15 defined by a gate line 110, a data line 120, and a power line 130. The pixel disposed in the pixel region 15 includes a switching OTFT 140, a driving OTFT 160, a capacitor 150, and an organic emission element including a lower electrode 180 used as a pixel electrode. The pixel region 15 includes an emission region where light emission occurs and a non-emission region where light emission does not occur. The organic electroluminescent (EL) device, which emits light, is disposed in the emission region. The switching OTFT 140, the driving OTFT 160, and the capacitor 150 are disposed in the non-emission region.
The switching OTFT 140 includes a gate electrode 141 connected to the gate line 110, a semiconductor layer 170, and source and drain electrodes 143 and 145. The capacitor 150 includes a lower electrode 151 connected to the drain electrode 145 of the switching OTFT 140, and an upper electrode 155 that overlaps the lower electrode 151 and is connected to the power line 130.
The driving OTFT 160 includes a gate electrode 161 connected to the lower electrode 151 of the capacitor 150, the semiconductor layer 170, a source electrode 163 connected to the power line 130, and a drain electrode 165 connected to the lower electrode 180 through a viahole 167.
FIG. 2 is a sectional view taken along line II-II in FIG. 1. FIG. 2 illustrates only the organic emission element including the anode electrode 180, an organic layer 190 and a cathode electrode 195, the driving OTFT 160 used to drive the organic emission element, and the capacitor 150 of the pixel.
Referring to FIG. 2, the gate electrode 161, and the lower electrode 151 of the capacitor 150 are formed on a substrate 100. A gate insulator 101 covers the gate electrode 161, and the lower electrode 151 of the capacitor 150, and the substrate 100. The source and drain electrodes 163 and 165 and the upper electrode 155 of the capacitor 150 are formed on the gate insulator 101, and the semiconductor layer 170 is formed over the substrate 100.
The lower electrode 180 used as a pixel electrode, which will be interchangeably used to refer to an anode electrode, is formed on a protecting layer 103, and is connected to the drain electrode 165 through the viahole 167. An opening 185 formed in a pixel isolation layer 105 exposes a portion of the lower electrode 180. The organic layer 190 is formed on the lower electrode 180 inside the opening 185. Then, the upper electrode 195 used as a cathode electrode is formed over the substrate 100.
As mentioned above, when the conventional organic electroluminescent display device has a rear emission structure in which light emitted from an organic layer is emitted toward a substrate, a lower electrode used as an anode is a transmittance electrode and an upper electrode used as a cathode is a non-transmittance electrode. Furthermore, in order to allow light to transmit from the organic layer 190 through the substrate 100, the intermediate layers (103, 170, 101) are formed of a light-transparent material.
However, the manufacturing process of such a conventional organic EL device is complicated because many masking processes are required to form a gate electrode, source and drain electrodes, a viahole, an anode electrode, a pixel isolation layer, and the like.